1. Field of the Invention
The invention relates to a process for ethylene recovery in a recirculating gas process for preparing vinyl acetate, in which a substream of the ethylenic product stream is discharged and sent to a process for recovering or converting ethylene.
2. Description of the Related Art
Vinyl acetate is prepared in continuous processes with recycling of the purified product stream. In a heterogeneously catalysed gas phase process, ethylene reacts with acetic acid and oxygen over fixed bed catalysts which generally comprise palladium and alkali metal salts on a support material, and may additionally also be doped with gold, rhodium or cadmium.
The ethylene, oxygen and acetic acid reactants are reacted in an exothermic reaction generally at a pressure of from 8 to 12 bar and a temperature of from 130° C. to 200° C. in a fixed bed tubular reactor to give vinyl acetate:C2H4+CH3COOH+0.5 O2=>CH3COOCH═CH2+H2O
The ethylene conversion is about 10%, the acetic acid conversion from about 20 to 30% and the oxygen conversion up to 90%.
A problem in this reaction is that inert substances are introduced via the ethylene and oxygen reactants, and are difficult to remove, but increasingly reduce the selectivity of the re-action when they are recycled into the reactor and accumulate in the circulating gas. The inerts nitrogen and argon are introduced via the oxygen, and ethane and to a slight extent methane via ethylene. These inerts have to be removed continuously from the system, otherwise their accumulation would inhibit the re-action. Since ethylene constitutes the majority of the recycle gas mixture at from 60 to 70% by volume, the removal of inerts is generally accompanied by a distinct loss of ethylene of from about 1 to 4% by volume of the amount fed. In addition, side reactions form carbon dioxide and further by-products such as methyl acetate and ethyl acetate.
Owing to the incomplete conversion of the reactants, the gaseous product stream is worked up in a continuous process: in a recycle gas scrubber operated with acetic acid, the vinyl acetate target product is washed out of the recycle gas and worked up in subsequent distillation processes. The vinyl acetate-free recycle gas is sent through a gas compressor to the acetic acid saturator and subsequently to the reactor. In order to reduce the CO2 by-product, a portion of the vinyl acetate-free recycle gas is discharged on the pressure side of the recycle gas compressor and sent to a water scrubber. Subsequently, a small fraction is sent to incineration for inerts discharge, and the remainder is passed into a CO2 absorption column and then sent back to gas recirculation in CO2 free form.
The inerts discharged by means of removal of ethylene from the recycle gas prevents accumulation of ethane, methane, argon and nitrogen in the recycle gas stream. The amount of the inert stream discharged is controlled depending on the ethylene concentration in the recycle gas. When the amounts discharged are too small, the inerts become concentrated in the recycle gas and the ethylene concentration in the recycle gas falls. However, the ethylene selectivity increases with the ethylene content of the recycle gas. The higher the ethylene content in the recycle gas, i.e. the more ethylene comprising inerts from the recycle gas is removed and “fresh” ethylene is supplied, the better the ethylene selectivity is. However, from a certain proportion, a more extensive discharge of ethylene comprising inerts is uneconomic, since each additional ton of vinyl acetate monomer has to be paid for with a disproportionately high fraction of discharged ethylene comprising inerts. Since ethylene is expensive, the recovery of ethylene has the highest priority as a cost-lowering measure.
WO-A 01/00559 describes two common alternatives for ethylene recovery in vinyl acetate preparation by means of gas phase reaction of ethylene, acetic acid and oxygen.
Carbon dioxide is removed from the gas stream leaving the reactor which then comprises primarily ethylene, methane, oxygen, nitrogen and argon. At system pressure, the gas stream is passed into an absorption column and washed with vinyl acetate, and a mixture of methane, nitrogen, oxygen and argon is drawn off at the top of the column and sent to incineration. At the bottom of the column, vinyl acetate and ethylene are withdrawn, the gas mixture is decompressed and ethylene is removed from vinyl acetate. The ethylene is subsequently compressed and passed back into the reactor.
Disadvantages in this context are the energy-intensive decompression-compression step, and the fact that the inerts cannot be removed fully, and thus become increasingly enriched and distinctly lower the selectivity of the reaction.
In a further variant, to which WO-A 01/00559 is directed, the majority of the gaseous product stream is contacted at system pressure with acetic acid in an absorption vessel. At the top of the column, methane, nitrogen, oxygen and argon are re-moved, and a mixture of vinyl acetate, acetic acid and ethyl-ene is drawn off at the bottom of the column. This mixture is contacted in a gas scrubber with the remaining fraction of the gaseous product stream. The ethylene is drawn off at the top and recycled into the reactor; the vinyl acetate is obtained at the bottom of the column and sent to further workup. The decompression/compression step becomes unnecessary, but here too the inert gases accumulate increasingly in the recycle gas.
A similar process is the subject matter of U.S. Pat. No. 3,714,237, in which the gaseous stream is likewise worked up by scrubbing with acetic acid, vinyl acetate is removed, and the residual gas is recycled into the reactor after the carbon dioxide has been washed out. Here too, the inert gases accumulate increasingly in the recycle gas.